It has become evident in the last several years that mutation rate is under genetic control. In bacteria and yeast the frequency of induced mutations can be either increased or decreased by blocking one or another pathway of DNA repair. This project is designed to determine the relationship between DNA repair and mutagenesis in Drosophila melanogaster. Two approaches are being taken: (1) A mutant which increases the mutation frequency (a mutator) has been identified, mapped, and characterized; and (2) the interaction of DNA repair defective mutants and hybrid dysgenesis (naturally occurring mutators) has been observed in double mutant combinations.